Apoptosis, the process of programmed cell death, is an essential biological process for tissue homeostasis. In mammals, it has been shown to regulate early embryonic development. Later in life, cell death is a default mechanism by which potentially dangerous cells, e.g., cells carrying cancerous defects, are removed. Several apoptotic pathways are known. One of the most important apoptotic pathways involves the Bcl-2 family of proteins which are key regulators of the mitochondrial (also called “intrinsic”) pathway of apoptosis. See Danial and Korsmeyer, Cell 116:205-219 (2004). The structural homology domains BH1, BH2, BH3 and BH4 are characteristic of Bcl-2 family proteins. The Bcl-2 family of proteins can be further classified into three subfamilies depending on how many of the homology domains each protein contains and on its biological activity, i.e., whether it has pro- or anti-apoptotic function.
The first subgroup of Bcl-2 proteins contains proteins having all four homology domains, i.e., BH1, BH2, BH3 and BH4. Their general effect is anti-apoptotic, that is to preserve a cell from starting a cell death process. Proteins such as Bcl-2, Bcl-w, Bcl-xL, Mcl-1, and Bfl-1/A1 are members of this first subgroup. Proteins belonging to the second subgroup of Bcl-2 proteins contain the three homology domains BH1, BH2, and BH3, and have a pro-apoptotic effect. The two main representative proteins of this second subgroup are Bax and Bak. The third subgroup of Bcl-2 proteins is composed of proteins containing only the BH3 domain and members of this subgroup are usually referred to as “BH3-only proteins.” Their biological effect on the cell is pro-apoptotic. Bim, Bid, Bad, Bik, Noxa, Hrk, Bmf, and Puma are examples of this third subfamily of proteins. The exact mechanism by which the Bcl-2 family proteins regulate cell death is not entirely known. In one hypothesis of regulation of cell death by Bcl-2 family proteins, the BH3-only proteins are further categorized as either “activator,” e.g., Bim and Bid, or “sensitizer,” e.g., Bad, Bik, Noxa, Hrk, Bmf, and Puma, proteins depending on their regulatory function.
One of the keys to tissue homeostasis is achieving a balance in the interactions among the three subgroups of Bcl-2 proteins in cells. Studies have elucidated the mechanisms by which pro-apoptotic and anti-apoptotic subgroups of Bcl-2 family proteins interact to allow a cell to undergo programmed cell death. After receiving intra- or extra-cellular signals in cells, post-translational or transcriptional activation of BH3-only proteins occurs. The BH3-only proteins are the primary inducers of an apoptotic cascade that includes, as one step, the activation of the pro-apoptotic proteins Bax and Bak on the mitochondrial membrane in cells. Upon activation of Bax and/or Bak that are either already anchored to the mitochondrial membrane or migrate to this membrane, Bax and/or Bak oligomerize to result in mitochondrial outer membrane permeabilization (MOMP), the release of cytochrome C, and downstream activation of effector caspases, to ultimately result in cell apoptosis. Some researchers hypothesize that certain BH3-only proteins, e.g., Puma, Bim, Bid, are “activators” in that these proteins directly engage pro-apoptotic proteins Bax and Bak to initiate MOMP, while other BH3-only proteins, e.g., Bad, Bik and Noxa, are “sensitizers” and induce Bax and Bak oligomerization indirectly by binding anti-apoptotic proteins, e.g., Bcl-2, Bcl-xL, Bcl-w, Mcl-1, and displacing and “freeing-up” the “activator” BH3-only proteins, which subsequently bind to and activate pro-apoptotic proteins, e.g., Bax, Bak, to induce cell death. Other research suggests that anti-apoptotic proteins engage and sequester Bax and Bak directly and all BH3-only proteins regulates this interaction by binding to anti-apoptotic proteins, e.g., Bcl-2, Bcl-xL, Bcl-w, Mcl-1, which results in the release Bax and Bak. See Adams and Cory, Oncogene 26:1324-1337 (2007) and Willis et al., Science 315:856-859 (2007). Although the exact interactions through which the anti- and pro-apoptotic Bcl-2 family proteins regulate apoptosis remain under investigation, there is a large body of scientific evidence to show that compounds which inhibit the binding of BH3-only proteins to anti-apoptotic Bcl-2 family proteins promote apoptosis in cells.
Dysregulated apoptotic pathways have been implicated in the pathology of many significant diseases such as neurodegenerative conditions (up-regulated apoptosis), such as for example, Alzheimer's disease; and proliferative diseases (down-regulated apoptosis) such as for example, cancer, autoimmune diseases and pro-thrombotic conditions.
Down-regulated apoptosis (and more particularly the Bcl-2 family of proteins) may be involved in the onset of cancerous malignancy. Research has shown, for example, the anti-apoptotic proteins, Bcl-2 and Bcl-xL, are over-expressed in many cancer cell types. See Zhang, Nature Reviews Drug Discovery 1:101 (2002); Kirkin et al., Biochimica et Biophysica Acta 1644:229-249 (2004); and Amundson et al., Cancer Research 60:6101-6110 (2000). The effect of this deregulation is the survival of altered cells which would otherwise have undergone apoptosis in normal conditions. The repetition of these defects associated with unregulated proliferation is thought to be the starting point of cancerous evolution. Additionally, research has shown that BH3-only proteins can act as tumor suppressors when expressed in diseased animals.
These findings have made possible new strategies in drug discovery for targeting cancer. If a small molecule that could mimic the effect of BH3-only proteins were able to enter the cell and overcome the anti-apoptotic protein over-expression, then it could be possible to reset the apoptotic process. This strategy can have the advantage that it can alleviate the problem of drug resistance which is usually a consequence of apoptotic deregulation (abnormal survival). Therapeutic strategies for targeting Bcl-2 and Bcl-XL in cancer to restore cancer cell sensitivity and overcome resistance of cancer cells to apoptosis have been reviewed. See Adams et al., Science 281:1322 (1998) and Reed, Adv. Pharmacol. 41:501 (1997); Reed et al., J. Cell. Biochem. 60:23 (1996).
Platelets also contain the necessary apoptotic machinery, e.g., Bax, Bak, Bcl-xL, Bcl-2, cytochrome c, caspase-9, caspase-3 and APAF-1, to execute programmed cell death through the intrinsic apoptotic pathway. Although circulating platelet production is a normal physiological process, a number of diseases are caused or exacerbated by excess of, or undesired activation of, platelets. This suggests that therapeutic agents capable of inhibiting anti-apoptotic proteins in platelets and reducing the number of platelets in mammals may be useful in treating pro-thrombotic conditions and diseases that are characterized by an excess of, or undesired activation of, platelets.
Small molecule BH3-only protein mimetics such as ABT-737 and ABT-263 bind strongly to a subset of anti-apoptotic Bcl-2 proteins including Bcl-2, Bcl-w and Bcl-xL, and weakly to Mcl-1 and A1. These small molecules were tested in animal studies and demonstrated cytotoxic activity in certain xenograft models as single agents, as well as enhanced the effects of a number of chemotherapeutic agents on other xenograft models when used in combination. See Tse, C. et al., Cancer Res 68: 3421-3428 (2008) and van Delft, M. F. et al., Cancer Cell 10:389-399 (2006). These in vivo studies suggest the potential utility of inhibitors of anti-apoptotic Bcl-2 family proteins for the treatment of diseases that involve a dysregulated apoptotic pathway. ABT-199 (Venetoclax) is a potent Bcl-2 inhibitor that has been approved by the U.S. Food and Drug Administration for the treatment of chronic lymphocytic leukemia. See Cang et al., Journal of Hematology & Oncology 8:129 (2015) and Souers et al., Nature Medicine 19:202-208 (2013).
The natural expression levels of anti-apoptotic Bcl-2 family proteins members vary in different cell types. For example, in young platelets, Bcl-xL protein is highly expressed and plays an important role in regulating cell death (life span) of platelets. Also, in certain cancer cell types, the cancer cell's survival is attributed to the dysregulation of the apoptotic pathway caused by the over-expression of one or more anti-apoptotic Bcl-2 protein family members. In view of the important role for Bcl-2 family of proteins in regulating apoptosis in both cancerous and normal, i.e., non-cancerous, cells, and the recognized inter-cell type variability of Bcl-2 family protein expression, it is advantageous to have a small molecule inhibitor that selectively targets and preferably binds to one type or a subset of anti-apoptotic Bcl-2 protein(s), for example, to an anti-apoptotic Bcl-2 family member that overexpressed in a certain cancer type. Such a selective compound also may confer certain advantages in the clinical setting, by providing, for example, the flexibility to select a dosing regimen, a reduced on-target toxic effect in normal cells, among others, e.g., lymphopenia has been observed in Bcl-2 deficient mice. See Nakayama, K. et al. PNAS 91:3700-3704 (1994).
There is an ongoing need for small molecules that selectively inhibit the activity of one type or a subset of Bcl-2 proteins for the treatment of hyperproliferative diseases such as cancer.